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My MB42 fireplace with digital fireplace burner control (SIT Proflame) will not stay on after starting. It shuts off many times, and only on a 6th try or so, will stay on.

The controller has a single wire connection to the "pilot flame sensor."

I suspect the sensor, and want to replace it.

However, I am confused by this sensor, as it has just one lead. Watching Fire Place repair videos I learned that there are thermocouple and thermopile sensors, but mine seems to be neither.

It does not have the two leads of the thermopile, nor the single tube of the thermocouple.It just has a single connection to CN4 on the controller.

What is this single-wire flame sensor called? It's the one on the left. The one on the middle looks similar, but is slightly wider and provides the spark.

sensor on the left

Digital Fireplace Burner Control

A little more on the symptoms: when just started, will shut off repeatedly. Sometimes before the main burner switches on, but sometimes when main burner is already on.

Once the main burner has been on for at least a minute, it will stay on.

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  • pre-heating it with a lighter will usually allow it to stay on easier, in my experience.
    – dandavis
    Oct 12, 2018 at 16:46

2 Answers 2

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By the way, that single element is called a flame rod and has been used as a very safe flame sensor for many, many, years. Things to try; 1) connect a D.C. micro amp meter in series with the lead to this sensor. A flame signal reading above 2 UA. is what is needed. If you get the 2 UA reading be happy even though a higher reading may be recommended. 2) make sure that the pilot head is firmly grounded to the rest of the burner and to the ground wire from the control (rusted connections are not allowed). 3) IF you cannot get a flame signal reading above 2 UA try adding some ground area to the pilot head with any thing metal that you can attach to that metal pilot head and it must be within the flame ( you could try an alligator clip to see if more ground area is needed). If the added grounding area causes the meter reading to go down or be reduced , remove that piece you just attached. 4) make sure that the porcelain insulator is not cracked or broken.(buy a spare and keep on hand). These are some things to try. Flame rods are tricky and can present problems without a very stable flame.

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  • The way a flame rod works is; an A/C voltage is applied to the rod. In the presence of a natural gas flame, the voltage passes through the flame in 1 direction only, producing a half wave rectification DC voltage. The strenght of the voltage through the flame to the ground surface is measured in micro amps. This is the flame signal strength On all gas appliances I serviced both "big and small" the normal signal strength was supposed to be About 2 UA' To change the signal strength up or down you would add or reduce the amount of grounding surface exposed to the flame.
    – d.george
    Oct 13, 2018 at 10:53
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That's a fairly typical flame sensor. A common remedy is to polish it up a bit with steel wool. They develop a carbon coating that inhibits sensitivity. Yours looks a bit grungy, and judging by your symptoms I'd guess that cleaning or replacing it will resolve the issue.

A flame sensor "senses" a weak DC signal from the AC power sent to the ignitor which via the phenomenon of flame rectification in which the polarity of power sent through a flame is rectified to DC, flame is a poor conductor so the signal is mere micro amps .3 to .5 usually and the flame sensor is nothing more than a rod insulated [b]y a ceramic base so it is not in contact with anything....

https://www.quora.com/How-does-a-flame-sensor-work

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  • Thanks. This is a long running problem, and despite many cleaning attempts it never went away. Will give it one more try though, and report back.
    – Bram
    Oct 11, 2018 at 18:04
  • So is the flame supposed to gap the bridge from sensor to igniter, or from sensor to pilot nozzle? I guess oxidation on those parts have influence as well.
    – Bram
    Oct 11, 2018 at 18:31
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    It senses the flame from the nozzle (or burner), in order to ensure that unburned gas isn't filling up your house. If you're asking about the actual electrical current path, I'm not sure.
    – isherwood
    Oct 11, 2018 at 18:41
  • @isherwood -- the path (on the dead furnace board I have on my bench at least) is from the AC mains, through a small capacitor that stringently limits the current, to ground through the flame sensor, with a small circuit tapped off at the flame sensor connection that senses the rectified DC produced by the flame rectification effects.
    – ThreePhaseEel
    Oct 11, 2018 at 22:37

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